Polymer coating for powder-free elastic strands

ABSTRACT

The present invention is directed to a coated elastomeric sheet or strands having a polymeric coating applied to both sides. The powder-free polymer coating provides good release and anti-blocking properties to elastic strands, as well as adding to the machinability and end-use properties of the strands. The polymer-coated elastic strands are useful in the manufacture of diapers and other disposable articles requiring flexible elastic-lined openings.

The benefit of the earlier filing date of provisional application No. 60/467,382 is claimed.

FIELD OF THE INVENTION

The present invention relates to the use of a polymeric coating on powder-free natural or synthetic rubber elastomeric strands and sheets. The polymer coating provides good release and anti-blocking properties to the elastic strips, as well as adding to the machinability and end-use properties of the strands.

BACKGROUND OF THE INVENTION

Natural and synthetic rubber is an elastic material that is very tacky and has a low glass transition temperature (Tg). Because of its tackiness, the material adheres to itself and other materials during the manufacturing process, especially prior to curing. A surface coating must be applied to the elastic material to prevent blocking, which is the adhering together of two rubber surfaces, and also to serve as a release coating to prevent the rubber from sticking to objects during manufacture.

In current practice, a powder, such as talc, calcium carbonate, or starch is coated onto the elastomeric strands, to prevent blocking, and serve as a release coating. The powder also makes cured rubber elastic strands slippery, helping to prevent entanglement of the strands for ease of assembly into end-use products such as baby diapers.

Unfortunately, the presence of powder on rubber strands can present problems to both the manufacturer of the rubber strands, and also the manufacturer of end-use products containing the rubber strands. First, the powders are dusty and easily come off. Air-borne powder contaminates the air, substrate surfaces, and machinery surfaces. The dust is an explosion hazard, and build-up on machinery can result in downtime for cleaning. The powders also tend to adsorb proteins found in natural rubber latex, then when the powder contacts skin or is inhaled, it can cause allergies and other negative effects.

The presence of powder on elastic strips is detrimental to the bonding of those strips to manufactured articles during the end-use assembly process. Generally hot-melt adhesives are used to bond the rubber strands to substrate surfaces. The presence of powder on the strands hinders direct contact between the adhesive and the rubber, decreasing bond efficiency.

Polymer coatings have been used in formed natural and synthetic rubber articles, such as gloves, catheters, condoms and balloons. Polymeric mold release agents have been disclosed in the art for use in making latex gloves. Formulated polychloroprenes are described in EP 0 640623, Urethane dispersions in U.S. Pat. No. 5,534,350, and styrene/acrylates containing silicone in U.S. Pat. Nos. 5,993,923; 5,691,069; 5,700,585; and 5,712,346. Copending U.S. patent application Ser. No. 09/400,488, and copending U.S. patent application, submitted Sep. 15, 2000 describe the use of star polymers as inner coatings for latex gloves. Copending U.S. patent application Ser. No. 09/790,093 describes the use of a polymer as a release coating for latex glove manufacture.

There is a need for a powder-free coating providing good release and anti-blocking properties for natural and synthetic rubber formed as a sheet or strand. The elastomeric material is often cut into strands and used in making end-use articles.

Surprisingly a polymeric coating applied to natural or synthetic rubber sheets provides excellent release and anti-blocking properties. Additionally, the polymer coating improves the strength of the elastomeric material during the manufacturing process. The polymer coating is also advantageous in that it bonds well to hot-melt adhesives typically used in attaching the elastic strands to a finished product.

SUMMARY OF THE INVENTION

It is an object of the invention to replace the powders currently used in the formation of natural and synthetic rubber sheeting and rubber strands.

It is an object of the invention to provide a polymer coating for natural or synthetic rubber sheeting or strands. The polymer coating must provide excellent release and anti-blocking properties.

It is an object of the invention to provide elastomeric strands with improved bonding characteristics during assembly, allowing for less adhesive use than needed with powdered rubber sheeting.

It is an object of the invention to reduce equipment maintenance due to the build-up of powder during the manufacture of elastomeric strands, and the assembly of the strands into products.

It is an object of the invention that the polymeric coating can stretch without coating delamination. Further, the coating should be deliverable from an aqueous solution, and be resistant to washing.

It is an object of the invention to improve the machinability of the elastomeric strands.

These objects are provided for by the current invention directed to a coated elastomeric sheet comprising a natural or synthetic rubber sheet having directly deposited on both sides of said sheet a polymer composition, the polymer composition comprising a high Tg polymer, having a Tg of greater than −10° C.

The invention is also directed to a method of making a coated elastomeric sheet comprising:

-   -   a) extruding or calendaring an elastomeric sheet of natural or         synthetic rubber;     -   b) forming an aqueous polymer composition comprising a high Tg         polymer, wherein said polymer comprises from 0.5 to 20 percent         by weight of the polymer composition;     -   c) applying said polymer composition to the elastomeric sheet;     -   d) curing said polymer coated elastomeric sheet.

The invention is further directed to an article having polymer coated elastomeric strands attached near one or more openings in the article.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, a rubber sheet or strand, and an elastic or elastomeric sheet or stand, are all terms meant to denote an elastic natural or synthetic rubber sheet. The sheet may be cut into smaller sheets or strands. The sheets or strands are generally flat, and are different than formed rubber objects such as gloves, condoms, catheters, and balloons that are formed on a mold and have the three-dimensional shape of the mold. The elasomeric strands are coated on both sides with the polymer coating composition of the invention.

The natural or synthetic rubber sheet of the invention is a low-Tg, tacky polymeric material. Examples of such materials include, but are not limited to, butyl rubber, natural latex rubber, polyisoprene, polyvinyl chloride, neoprene, nitrile, viton, styrene butadiene copolymers, polyurethanes, interpenetrating polymer network emulsion polymers, or combinations of these.

The elastomeric sheet is generally formed by extruding or calendaring. The elastomeric sheet can then be cut into strands following formation.

The elastomeric sheet or strands are coated with an aqueous polymer composition. The aqueous polymer composition is a dispersion of one or more polymers and optional adjuvants.

The polymer used in the invention is a high Tg polymer or copolymer. A high Tg polymer in the context of the invention is one having a Tg from −10 to 120° C., preferably from 20 to 110° C., more preferably from 25 to 90° C., and most preferably from 40° C. to 80° C. Polymers useful in the present invention are those formed from ethylenically unsaturated monomers by means known in the art, or mixtures thereof. Particularly useful polymers include (meth)acrylic copolymers, vinyl acrylics, polyvinyl acetate, vinyl copolymers, ethylene-vinyl acetate copolymers, and polyurethanes. Optionally, a high Tg copolymer could also contain a low energy monomer, and adhesion promoter. In one embodiment, the polymer is a water-borne acrylic polymer having a Tg of 40 to 80° C. The polymer composition may also contain a blend of two or more different polymers.

The polymer of the present invention may be formed by means known in the art, such as emulsion polymerization, solution polymerization, and suspension polymerization. The process may be a batch, semi-batch, or continuous process.

The polymer composition may be simply a polymer in an aqueous medium. It may be two or more different polymers dispersed in water. More preferably, the polymer composition also contains adjuvants useful in stability of the polymer composition, and useful in the end-use application. Useful adjuvants include, but are not limited to dispersants, adhesion promoters, microspheres, rheology modifiers, surfactants, crosslinking agents, biocides, low surface energy compounds, and fillers, and anti-foaming agents.

A dispersant can be used to help stabilize the polymer particles, especially when the polymer composition is diluted before application to the rubber sheet. While not being bound to any theory, it is believed that the addition of a dispersant to a polymer composition results in stabilization of the polymer. It also helps in forming a uniform and continuous film on the elastic strand. One or more dispersants are preferably present in the polymer composition at from 2 to 50 percent by weight, and more preferably 5 to 30 percent by weight, based on the weight of the polymer in the polymer composition. The dispersant may be a polymer, a non-polymer, or a mixture thereof. Non-polymeric dispersants useful in the present invention include, but are not limited to, anionic, cationic, nonionic, and amphoteric surfactants. Polymeric dispersants include amphiphilic linear and star copolymers. Linear polymers useful in the present invention include, but are not limited to, poly(vinyl alcohol); partially hydrolyzed poly(vinyl alcohol); poly(acrylic acid); poly(methacrylic acid); copolymers of acrylic acid and/or methacrylic acid with compatible ethylenically unsaturated monomers such as alkyl esters of (meth)acrylic acid, hydroxyalkyl esters of (meth)acrylic acid, alpha-methyl styrene, styrene, and derivatives thereof, vinyl acetate, crotonic acid, esters of crotonic acid, and acrylamide, and derivatives thereof. Other suitable linear polymeric dispersants include but are not limited to poly(maleic acid) and copolymers of maleic acid with compatible ethylenically unsaturated monomers such as mono- and diesters of maleic acid, (meth)acrylic acid, alkyl esters of (meth)acrylic acid, hydroxyalkyl esters of (meth)acrylic acid, alpha-methyl styrene, styrene, and derivatives thereof, vinyl acetate, crotonic acid, esters of crotonic acid, and acrylamide, and derivatives thereof. Still other suitable linear polymeric dispersants include, but are not limited to, polystyrene sulfonates, which are typically obtained by sulfonating poly(styrene) or copolymers of styrene with compatible ethylenically unsaturated monomers including, but not limited to, (meth)acrylic acid, esters of (meth)acrylic acid, maleic acid, and mono- and diesters of maleic acid; condensates including but not limited to naphthalenesulfonic acid-formaldehyde condensate and melamine-formaldehyde condensate. Certain natural or naturally derived polymers useful in the present invention include but are not limited to tannins, lignins, lignosulfates, alginates, dispersed or soluble starches and modified starches, and cellulosic polymers.

Star or radial polymers, as used herein, is intended to describe polymers that have three or more polymeric arms emanating from a central core. These polymers can be prepared by various polymerization procedures such as anionic, cationic, and free radical mechanisms. The star polymers are usually formed by using either multifunctional initiators, multifunctional chain transfer agents, or multifunctional coupling agents. The star polymers have unique properties including: low viscosities in solution due to their compact structure and high melt viscosities due to extensive entanglements relative to their linear coatings.

The polymer composition may also contain microspheres. Microspheres are useful in reducing the surface contact area, improving both the release and anti-blocking characteristics. The microspheres have diameters below 60 microns, preferably from 5 to 40 microns, and most preferably from 10 to 30 microns. The microsphere may be made of any material that is harder than the article being coated. Examples of microspheres useful in the present invention are those made of polyamides such as nylons, polymethylmethacrylate, polystyrene, polyethylene, polypropylene, polytetrafluoroethylene, polyesters, polyethers, polysulfones, polycarbonates, polyether ether ketones, and other polymers and copolymers, silica, and microcrystalline cellulose. If present, the microspheres are present in the polymer composition at from 0.5 to 30 percent by weight, and most preferably at from 1 to 15 percent by weight, based on the weight of the polymer A rheology modifier is optionally present in the polymer composition. The rheology modifier is used to control the viscosity of the composition for ease of use in different manufacturing processes and equipment, and to control the uniformity and thickness of the coating. Rheology modifiers useful in the present invention include, but are not limited to cellulosics such as hydroxyethylcellulose, cationic hydroxyethylcellulose, such as polyquaternium-4 and polyquaternium-10, hydrophobically modified hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and hydroxypropylcellulose; dispersed or soluble starches or modified starches; and polysaccharide gums such as xanthan gum, guar gum, cationic guar gum such as guar hydroxypropyltrimonium chloride, and locust bean gum. Other suitable rheology modifiers include but are not limited to acid soluble copolymers, surfactants, etc. The rheology modifier is typically added at from 1 to 30 percent by weight, and preferably from 2 to 20 percent by weight, based on the polymer solids.

The release composition of the present invention is made by combining each of the ingredients to form an aqueous dispersion, by means known in the art.

Adhesion promoters may optionally be added to the polymer composition, and may be needed for some polymers, to add charge and increase the amount of polymer picked up by the elastic strands.

The polymer coating composition may be made by combining each of the ingredients to form an aqueous dispersion

The polymer composition is preferably applied as an aqueous dispersion by spraying or dipping onto the rubber strands. The polymer composition may also be applied by other means, such as, for example, by brushing. The dipping or spray can easily be incorporated in current rubber strand production processes. The polymer composition may be diluted prior to application, to optimize the application process. Elastic strands may be dipped into a polymer composition containing from 0.5 to 20 percent by weight of polymer solids, and preferably from 3 to 10 percent by weight of polymer solids. The aqueous polymer composition is then dried to form a continuous polymer film on the elastic strand, followed by curing of the elastomer. It was found that the polymer coating composition adhered permanently to the rubber elastic strip, and did not peel off or crack, even under repeated stretching and high temperatures.

The polymer coating on each side of the rubber sheet or strands is preferably from 0.1 to 10 percent by weight of the rubber sheet, more preferably from 0.5 to 5 percent, and most preferably from 1 to 3 percent.

The polymer coated rubber strands have good release properties, good anti-blocking properties, good bonding properties to the elastic surface, and good bonding with hot melt and other adhesives in end-use manufacturing assembly. The polymer coating might provide improved machinability of the rubber strands by improving tensile strength, thus helping to prevent breakage during the manufacturing process.

Preferably the polymer coating of the invention is safe for direct skin contact.

The powder-free polymer coated elastic strips are especially useful in personal disposable items, such as baby diaper diapers; adult incontinent articles; hospital gowns, boots and caps; sanitary napkin.

One particular advantage of the present invention is that the polymer-coated elastic strips show good bonding with standard hot-melt adhesives. Hot-melt adhesives are often used to attach the elastic strip to a substrate, such as in the use of the elastic strip at the leg opening of a disposable diaper. The presence of a powder coating in the current process hinders the contact between the adhesive and rubber, decreasing bonding efficiency. As a result, high amounts of hot melt adhesive are required. The bonding between the polymer-coated elastic strip and adhesive is excellent, resulting in much better bonding. This results in the use of much less adhesive in the assembly process.

During the assembly process, the elastic strands are often in direct contact with the adhesive application head. The presence of powder can build-up on, and clog the head, shutting down a production line. The polymer-coated elastic strands of the present invention overcome this process problem.

Polymer-coated elastic strands may also be used in products such as medical and industrial protective wear such as, for example, gloves, and facemasks. The powder-free polymer coated elastic can be made into strands, threads, and tapes useful in, but not limited to, said listed products.

EXAMPLES

The following examples are presented to further illustrate and explain the present invention and should not be taken as limiting in any regard.

Example 1 Polymer Composition

A polymer coating composition was prepared containing 1.5 percent of linear copolymer of 45 parts methyl methacrylate, 10 parts methacrylic acid, and 45 parts butyl acrylate, 1 percent by weight of NACRYLIC 6408 (meth)acrylic emulsion, 0.25% by weight of polymethylmethacrylate-co-ethylene glycol di-methacrylate copolymer beads having average diameter of 8 micron (ALDRICH 46,316-7), and 0.075 percent by weight of KELTROL T xanthan gum. The dispersion was stable.

Example 2 Polymer Composition

A polymer coating composition was prepared containing 1.5% of a heteroarm star copolymer, 1 percent by weight of NACRYLIC 6408 (meth)acrylic emulsion, 0.25 percent by weight of polymethylmethacrylate-co-ethylene glycol di-methacrylate copolymer beads having average diameter of 8 micron (ALDRICH 46,316-7) and 0.075% by weight of KELTROL T xanthan gum. The dispersion stable.

Example 3 Polymer Composition

A polymer coating composition was prepared containing 1.5% of random star copolymer of Example 2, 1% by weight of NACRYLIC 6408 (meth)acrylic emulsion, 0.25% by weight of polymethylmethacrylate-co-ethylene glycol di-methacrylate copolymer beads having average diameter of 8 micron and oil adsorption of 55 mL/100 g (ALDRICH 46,316-7) and 0.075% by weight of KELTROL T xanthan gum. The dispersion was significantly more stable than in Example 6 and practically no settling was observed at the bottom of the container. The dispersion was used for coating rubber articles, which exhibited excellent donning properties.

Example 4 Coating of Polymer Onto Rubber Strands

Rubber elastic strands, from Fullflex Elastomerics were washed in water containing a small amount of detergent to remove the talc powder on the surface. The rubber strands were then rinsed thoroughly in distilled water. The washed elastic strip was then dipped into a diluted polymer composition of Example 3. The polymer solids level of the diluted emulsions was 5 percent, 10 percent, and 15 percent. Following dipping, the coated elastic strips were placed into a 60° C. oven for 3 hours to dry. After drying, all of the elastic strips were found to have smooth surfaces. No cracking or peeling of the polymer coating was seen, even after repeated stretching of the strands.

It was found that dipping in a 5 percent polymer coating produced good results. The higher levels were used to check for problems that might be found with cracking, peeling, and bonding with hot melt adhesive.

Example 5 Adhesion Testing

Elastic samples of Example 3 were tested for adhesion of a styrene block copolymer hot-melt adhesive (DM 727 from National Starch and Chemical Company) to the elastic surface. The Kanebo adhesion test was used. Each elastic strip was attached to Kanebo paperboard across the line direction with tape. A bead of DM727 was applied across the elastic. The pressure was applied to the adhesive bead to form bonding with elastic strand. The test was performed with the following conditions:

-   -   Adhesive: DM727     -   Coating weight: 26.3 g/min     -   Application temperature: 155° C.     -   Open time: 5 seconds     -   Set time: 0.5 seconds     -   Pressure: 0.2 Kg/cm³     -   Line speed: 30 m/min

The bond strength of the elastic to adhesive was measured on an Instrom at 23° C. The paperboard was clamped by the lower jar, and the elastic was clamped by the upper jaw. 100 mm/min crosshead speed was used, and the bond strength was measured as the maximum pull force. 7 to 8 specimens of each sample were tested, and the bond strength reported as the average of all specimens. The results are found in Table 1. TABLE 1 Elastic strip sample Bond Strength (N) Unwashed (Comparative) 0.94 Washed (Comparative) 1.16  5% solids polymer coating 1.09 10% solids polymer 1.10 coating 15% solids polymer 0.99 coating The results in Table 1 show that talc effects the bond strength. The polymer-coated strands exhibited much higher bond strength than the talc-coated samples. 

1. An elastomeric strand or sheet having coated thereon an aqueous polymer composition comprising a polymer or having a Tg greater than −10° C.
 2. The strand or sheet of claim 1 wherein said polymer has a Tg of from 20 to 110° C.
 3. The strand or sheet of claim 2 wherein said polymer has a Tg of from 25 to 90° C.
 4. The strand or sheet of claim 3 wherein said polymer has a Tg of from 40 to 80° C.
 5. The strand or sheet of claim 1 wherein the polymer is selected from the group consisting of acrylic copolymers, methacrylic copolymers, vinyl acrylics, polyvinyl acetate, vinyl copolymers, ethylene vinyl acetate copolymers, polyurethanes and mixtures thereof.
 6. The strand or sheet of claim 1 wherein said polymer composition comprises a waterborne acrylic polymer having a Tg of from 40 to 80° C.
 7. The strand or sheet of claim 1 wherein said polymer composition comprises a dispersant comprises a star polymer.
 8. The strand or sheet of claim 1 wherein said polymer composition comprises microbeads.
 9. A method of making a coated elastomeric strand or sheet comprising: extruding or calendaring an elasctomeric sheet; forming an aqueous polymer composition comprising a polymer, applying said polymer composition to the elastomeric strand or sheet; and curing said polymer coated strand or sheet.
 10. The method of claim 9 wherein said polymer comprises from 0.5 to 20 percent by weight of the polymer composition.
 11. The method of claim 9 wherein said polymer composition is applied by spray or dipping.
 12. The method of claim 9 wherein said polymer composition comprises one or more adjuvants selected from the groups consisting of adhesion promoters, microbeads, and dispersants.
 13. An article comprising the strand or sheet of claim
 1. 14. The article of claim 13, which is a disposable garment.
 15. The article of claim 14, wherein said article is selected from the group consisting of diapers, adult incontinence articles, hospital gowns, booties, head/hair coverings, medical and industrial protective wear, gloves, and facemasks. 